 |  Perspectives in Nutrition, 5/e Gordon M. Wardlaw,
Ohio State University
Margaret W. Kessel,
Ohio State University
Trace Minerals
Chapter 12 Summary- Four of the trace minerals (iron, zinc, iodide, and selenium) have
an RDA. An Adequate Intake has been set for five minerals (copper, manganese,
chromium, molybdenum, and fluoride).
- Some trace minerals are difficult to detect in humans, and it is
often hard to determine the exact amount of a trace mineral in food. Deficiencies
were first observed in small, geographically isolated groups (e.g., selenium
deficiency in an area of China) or people nourished exclusively by total parenteral
nutrition that did not contain sufficient trace minerals.
- Iron is a critical component of hemoglobin, myoglobin, and cytochromes.
Iron acts as a cofactor for several enzyme systems. Two-thirds of the body's
iron is found in hemoglobin in red blood cells, where its job is to transport
oxygen from the lungs to the tissues. A prolonged low intake of iron can lead
to decreased production of red blood cells and a lack of oxygen being delivered
to the tissues. This condition is called iron deficiency anemia, which results
in fatigue and apathy, as well as decreased learning ability in children.
- The absorption of iron depends on the body's need for the mineral
and on the form of iron in food. The body cannot readily excrete excess iron,
but the body has evolved a mucosal block, which limits overabsorption. Heme
iron from animal foods is better absorbed than nonheme iron obtained primarily
from plant sources. The best sources of dietary iron are animal protein, including
beef and other dark meats, oysters, liver, and broccoli.
- Girls and women have a higher RDA for iron than men because of menstrual
blood loss. Infants and children who live in poverty are often iron deficient
because of a lack of heme iron in the diet.
- Iron toxicity occurs because of a genetic disorder called hemochromatosis,
which causes the overabsorption of iron. A common form of poisoning also occurs
among toddlers and young children who swallow a large number of iron pills.
Death can occur.
- Zinc functions as a cofactor for more than nearly 100 enzyme systems,
which are important for growth, sexual development, immune function, wound healing,
and taste. A zinc deficiency results in growth failure, loss of appetite, inadequate
mental function, a persistent rash, and decreased immune function. Zinc deficiency
in the United States and Canada is rare.
- Like iron, the best dietary sources of zinc are found in animal foods.
Need drives absorption. And like iron, there is a mucosal block in the intestinal
cells, which regulates the amount of zinc that can be absorbed. Calcium, copper,
and iron in supplement form can interfere with zinc absorption. The richest
source of zinc is oysters. Other animal proteins are excellent sources. Plant
sources are whole grains, peanuts, and legumes.
- Copper aids in iron absorption and mobilization from body stores.
Copper is responsible for the cross-linking in collagen formation and for nerve
cell myelination, and it acts as part of an enzyme that is a scavenger
for free radicals. A copper deficiency can result in a secondary iron deficiency.
Copper is found in liver, cocoa, legumes, and whole grains. The copper content
of the soil where a plant is grown affects the copper content of the plant food.
- Selenium acts as a cofactor for the enzyme glutathionine peroxidase,
which protects cells against destruction by hydrogen peroxide and free radicals.
In some instances, selenium can replace some of the need for vitamin E. Human
deficiency is rare in the United States and Canada. The selenium content of
the soil where a plant is grown greatly affects the selenium content of the
plant food. In a few areas in China where the soil is selenium poor, the inhabitants
experience selenium deficiency. Meat, eggs, fish, and shellfish are sources
of selenium. Plant sources include grains and plant seeds.
- Iodide forms part of the thyroid hormones, one being thyroxine.
Thyroid hormone controls the basal metabolic rate. A lack of dietary iodide
causes an enlarged thyroid gland, known as goiter. The iodide content of the
soil where a plant is grown greatly affects the iodide content of the plant
food. Iodide deficiency at one time was common in areas around the Great Lakes
in North America because the soil is iodide poor. Today, iodide deficiency in
Canada and the United States is virtually unknown because of the fortification
of table salt with iodide, but there is some concern about iodide status in
pregnant women.
- Fluoride exposure makes the tooth crystal resistant to dental caries,
and fluoride in saliva aids in the remineralization of damaged tooth surfaces.
Most North Americans receive fluoride from fluoridated drinking water and toothpaste.
- Chromium contributes to the action of insulin. Chromium is found
in meats and whole grains.
- Manganese functions in several important enzyme systems. Deficiency
is rare. Whole grains, legumes, and animal foods are reliable food sources.
- Molybdenum is found in several enzyme systems. Deficiency is rare.
Molybdenum is found in plant foods such as legumes and whole grains.
- Boron contributes to ion transport in cell membranes. Fruits, leafy
vegetables, nuts, and beans are sources.
- Nickel likely participates in amino acid metabolism. Nickel is found
in nuts, beans, and whole grains.
- Silicon is involved in bone formation. Root vegetables and whole
grains are sources.
- Arsenic likely participates in amino acid and DNA metabolism. Fish,
grains and cereal products are sources.
- Vanadium likely has insulin-like actions in the body. Shellfish
and mushrooms are sources.
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2002 McGraw-Hill Higher Education